I LOW TEMPERATURE METALS

same behavior. Cast metals and alloys generally exhibit somewhat lower toughness thanwrought products at the same strength levels, but are less prone to directional effects.The lower toughness of cast metals is attributable to the generally coarser grainstructure, segregation of alloys, intermetallic compounds and nonmetallic inclusionsaround grain boundaries (or around the primary solidification structure) and to castingdefects. Hot working generally eliminates these casting defects, refines and homogenizes the structure and improves the ductility and toughness.Sjmilarly, heat treatments can drastically affect both ductility and toughness atgiven strength levels by changing grain size and the size, shape and distribution ofphases and intermetallic compounds.As,should be evident from the preceding discussions, welding produces a cast structure and can exert a profound effect upon the ductility and ,toughness of metals. Thisis especially true at reduced temperatures. Welding introduces geometrical factorsthat promote brittle behavior, notches and stress concentrators. It also causes com; .plex stress distributions resulting from weld undercutting, changes in section thickness at the weld crown on one or both surfaces, and from slag entrapment, porosity, orweld cracks. Some of the metallurgical structures thus formed may be very notchbrittle and prone to crack propagation, especially if minute flaws occur in such’zones.Another possible source of weld embrittlement is contamination by atmosphericgases in contact with the hot solid or liquid metal in the weld zone. Titanium, forexample, can be severely embrittled by the absorption of oxygen and nitrogen during.welding. This metal and its alloys must be welded either in vacuum or inert atmospherechambers, or with inert atmosphere shielding applied to both sides of the weld jointwhen welding in air.Steel weldments may undergo severe reductions in ductility and become prone tocracking by absorption of hydrogen resulting from the decomposition of moisture in or‘ganic electrode coatings OF in the welding atmosphere. Special low-hydrogen electrodecoatings have been developed for the welding of steels subjected to critical serviceapplications. Post-welding stress-relieving heat treatments are often applied to weldments not only to relieve stresses but also to temper or to age brittle, heat-affectedzones to improve their toughness.TESTS FOR MATERIAL SELECTIONWhile the standard tensile test, if conducted at sufficiently low temperatures,shows the effect of reduced temperatures upon the behavior of metals, it is not by itself sufficient or satisfactory for use in selecting materials for low-temperature.applications. As described previously, the simple, uniaxial stress distribution, absence of stress concentrators, and low rate of loading, enhance the tendency towardductile behavior in the tension test. A number of other tests were consequently developed to evaluate the embrittlement of metals as influenced by metallurgical., mechanical,and physical conditions of the test. Practically all of these tests possessed at leasttwo features in common: stress concentrators and multiaxial stress distributions, bothof which result from notches located within the test section.The earliest tests (between the turn of the century and World War I) generallyin;olved impact since at that time high-speed loading was considered to be a majorfactor in brittle fracture. The keyhole and V-notch Charpy and the lzod impact testsare representative of the early tests used to evaluate the effects of low temperatureupon the behavior of metals.The V-notch Charpy impact test is the most widely used of the three tests and- 314 -IIIIII.1111III- II1IIIII

II1III.IIIIUi-.finds extensive use to this day. Here a 60 V-notch is machined across one face ofthe specimen. The bar is broken by being supported at both ends and struck by a pendulum-supported weight impacting the face opposite the notch. The energy absorbed inrupturing the bar is determined by measuring the loss in kinetic energy of the pendulum.The notched-bar impact test is generally considered to be qualitative in nature.Materials specifications incorporating notched-bar impact-test requirements have been-developed for many critical ,applications. Generally, requirements have been established on the basis of experience gained by correlating notched-bar impact properties withsimulated or actual service behavior of full-size or scale-model structures.8,9A group of researchers at Watertown Arsenal” has attempted to employ the V-notchCharpy impact test quantitatively in materials specifications covering low- and mediumalloy steels required for critically stressed applications at room and moderately lowtemperatures. The approach is based.upon the equivalence of low temperature and highstrain rate in causing transition from ductile to brittle fracture (Fig. 2c). From anestimation of the maximum strain rate: and lowest service temperature to which an engineering structure or part will be subjected, it is possible to calculate, or pick offfrom a special graph the temperature at which a V-notch Charpy impact-test specimenshould be tested to behave the same as regards ductile or brittle fracture as the partin question. Several Army Ordnance specifications based on this approach have sincewithstood the searching test of time.While the nocched-bar impact test is very useful for evaluating the toughness offorgings, plate, and bar stock, it is not applicable to very thin sheet materials such’as are often employed in aerospace vehicles and airborne pressure vessels. Anothertype of test specimen that has been employed to evaluate the brittle-fracture tendencyof heavy sections of steel, and more recently of sheet alloys, is a tensile test spe.cimennotched on both sides. In round specimens, the notch is circumferential.A variety of notches has been employed by various investigators, with stress concentrations ranging from 3 for mild notches to as high as 18 for severely notched specimens (ASTM and NASA standard specimens). At General Dynamics/Astronautics,.we havestandardized on a notched tensile-test specimen having a stress concentration factorof 6.3, since this specimen has been found to yield results that correlate with thefracture behavior of full-scale, thin-skinned cryogenic pressure vessels and with thebehavior of large specimens containing complex welded joints.l1,l2 With materialsthat are notch-tough, the effect of the biaxial stress distribution at the notchedsection is to increase the effective strength of the material, and the notched tensilestrength may range from 1.0 to approximately 1.5 times the smooth tensile strength.In notch-brittle or notch-sensitive steels, the effect of the notch is to induce premature brittle fracture, and the notched tensile strength will be less than the smoothtensile strength. The ratio of the two strengths thus serves as an index of the brittle .fracture characteristics of materials.8. A . Hurlich, ASTM Special Tech. Publ. No. 158, p. 262 (1954).9. A . Hurlich and A.F. Jones, Metal Progress 2,65 (1957).10. L.D. Jaffe et al., SAE Journal 59 (Mar. 1951).11. J.F. Watson, J.L. Christian, T.T. Tanalski, and A. Hurlich, ASTM SpecialTech. Publ. No. 302, p . 129 (1961).12. A. Hurlich, ASTM Special Tech. Publ. No. 287, p. 215 (1960).-315-I

More recently, with the development of the concepts of fracture mechanic ,' , it has become possible to employ notched tensile tests in a quantitative manner todetermine the critical crack-extension force, the critical crack length beyond whichcatastrophically rapid crack propagation ensues, or the stress level where a cracklikeflaw or defect will propagate to result in failure.Other tests of importance in evaluating the mechanical properties of materials atreduced temperatures include both notched and unnotched fatigue tests, bend tests (par:ticularly of weld joints) , and notched and unnorched tensile tests of fusion weldments.Notched and unnotched tensile fatigue, bend and impact tests can be readily performed over a range of temperatures down to the boiling point of liquid helium (-452OF)to evaluate the effect of low temperatures upon the brittle fracture characteristics ofmetals. For the sake of convenience and economy, low-temperature mechanical-propertytests are generally conducted at some of the following temperatures: -lOO F (dry ice(liquid hydrogen)and alcohol) ; -320 F (liquid nitrogen) ; or -423'F.Temperatures between room and -lOO F can be achieved and maintained constant byadding appropriate amounts of dry ice to the alcohol as may be needed. .A typical test facility for conducting tensile tests at temperatures down to thatof liquid hydrogen is shown in Fig. 5. The liquid hydrogen is contained in a doublewalled, vacuum-jacketed Dewar, surrounded by a liquid-nitrogen bath that .in turn isinsulated with polyurethane foam. Pull rods extend through Teflon-sealed holes in thebottom and top cover of the liquid-hydrogen Dewar. Hydrogen gas boil-off is ventedthrough a flexible steel hose attached to the cover. The test equipment is located ina room titted with a sealed sheet-steel ceiling pierced with vents fitted with explosion-proofed motors and fans to provide rapid air changes within the room. All electrical connections more than a few feet off the floor are explosion-proofed, while asheet-steel room having positive air pressure is erected round the console of the tensile machine since the many electrical connections in this unit could not be explosionproofed. More than 15 000 tests have been performed with liquid hydrogen in this facility at General Dynamics/Astronautics since 1959 without a single serious incident.A large outdoor liquid-hydrogen test facility is shown in Fig. 6. This installation includes a tensile-test facility (far left), larger bend and compression-test. fixtures (center) fitted with double-walled vacuum-jacketed liquid-hydrogen chambers,and a cryogenic test setup for cyclic fatigue testing of the large coupons of thinmaterials used for cryogenic propellant storage. In all of these test fixtures, loading is applied by means of hydraulic load cells calibrated to read loads directly.13. G.R. Irwin, Naval Res. Lab. Report 4763 (1956).14. G.R. Irwin, J . A . Kies, and H.L. Smith, Proc. A S T M X (1958).- 316 -IIIIIIIIIIIIIIIIIII

IIIIIIIIIIIIIIIIIIIMETALS FOR CRYOGENIC APPLICATIONSReferences 15 to 20 give data on a variety of low-temperature materials. A verycomprehensive compilation of data on the mechanical and physical properties of a largeof metallic alloys, as well as nonmetallic materials, is included in the "Cryo. . numbergenic Materials Data Handbook." This handbook was initially compiled by the CryogenicEngineering Laboratory, National Bureau of Standards, Boulder, Colorado, but it now'being kept up to date by the Martin Co;, at Denver, under contract to the AeronauticalSystems Division, Air Force Systems Command, Wright-Patterson Air Force Base, Dayton,Ohio.Typical of such data is the accompanying table of results obtained at GeneralDynamics/AstronauticS on sheet nickel alloys.Figure 7 lists the more important steels, alumlnwn, nickel, and titanium-base alloys that are suitable for critically stressed applications at reduced temperatures.It places them in order corresponding to the lowest temperatures at which they may bereliably used. This method of presentation, borrowed from J.M. Hedge," provides aready reference to the comparative usefulness of metallic alloys at low temperatures.It should be borne in mind, however, that high toughness is not always an essentialrequirement for low-temperature applications. Many parts subjected to extreme subzerotemperatures are not subjected to high stresses, multiaxial stress distributions, orimpact loading. Magnesium-alloy castings, which are extremely notch brittle, haveserved very satisfactorily as pump housings, valve bodies, and in other applicationsat liquid-hydrogen temperatures. The alloys listed in Fig. 7 were selected on thebasis of their suitability for critically stressed applications where notches, sharpchanges in section, complex stress distributions, and high rates of loading may be involved in addition to the reduced operating temperatures. Another factor that enteredinto their placement in Fig. 7 was consideration of the alloys'*weldability,and toughness of the weld joints at reduced temperatures.The various alloys will be briefly discussed in the following sections where thelow-temperature ranges are divided into four classifications for uses down to -5OoF,.to -150 F, to -320 F, and below 320 F.-Metals for use to -50'3'.The temperature range from ambient down to -50 F is.ofinterest because it essentially encompasses the minimum temperatures encountered onthe earth's surface - as well as the boiling temperatures of ammonia, propane and freon,materials that are of considerable interest to the chemical processing and refrigeration industries.IMost of the standard constructional carbon steels such as the ASTM A7, A36, orA373 grades cannot be reliably used at temperatures down to -50 F. These steels are15. M.P. Hanson, G.W. Stickley, and H.T. Richards, ASTM Special Tech. Publ. No. 287,p. 3 (1960).16. J.F. Watson, J.L. Christian, and J. Hertz, Electro-Technology (Sept.-Nov. 1961).17. J.L. Christian and A . Hurlich, ASD-TDR-62-258, Part I1 (1963).18. J.L. Christian and J.F. Watson, Advances in Crvogenic Engineering, Vol. 7p. 490 (Plenum Press, New York, 1962).19. J.L. Christian, ASD-TDR-62-258 (1962).20. 'J.E. Campbell and L.P. Rice, ASTM Special Tech. Publ. No. 287 (1960).21. J.M. Hodge, ASTM Special Tech. Publ. No. 302, p. 96 (1961).- 317 -

not subject to notched-bar impact test requirements and may or may not be aluminumkilled to develop fine grain sizes. Their cooling rates during.processing are notcontrolled. They may vary from tough to brittle at ambient temperatures. There are,however, several fine-grained carbon steels available for low-temperature applicationsdown to -50 F. These are the ASTM A334-61TY A333-61T and A420-61T grades that are required to meet -50 F notched-bar impact-test requirements. In addition, silicon-killedfine-grained carbon steels of the ASTM A-201 and A-212 grades have good toughness properties down to -50 F, but are not required to meet notched-bar impact tests except whenspecified to meet the test requirements of A-300. . In the latter case, they may be andare widely used in refrigeration and transport equipment.Quenched and tempered low-alloy steels are, of course, applicable at temperaturesdown to -50 F, and many of them.are suitable for use at temperatures down to -lOO F or-150 F, but these will be discussed more fully in the next section.Practically all aluminum and titanium alloys may be used in critically stressedapplications at temperatures down to -SOOF, except for some of the highest-strengthaluminum alloys such as 7178-T6 and 7075-T6. These are not recommended, especiallywhere sharp changes in section, complex stress distributions or impact loads are involved. Similarly, the all-beta 13V-llCr-3Al-titanium alloy (12OVCA) and the 8 Mntitanium alloy tend to be notch-brittle at moderately reduced temperatures.Nickel and copper-base alloys are virtually all suitable for use at temperaturesdown to -50 F, and generally much lower.Metals for use to -150'F.Low-alloy steels suitable for use at temperaturesdown to -150 F fall into two categories: quenched and tempered steels having essentially fine-grained, tempered, martensitic microstructures, and nickel-alloyedcferritic steels. Most of the lower carbon (0.20 to 0.35% C) low-alloy steels havingsufficient hardenability to achieve martensitic microstructures through the sectionthickness when either water- or oil-quenched are, after tempering at appropriate temperatures, sufficiently tough for most critical service applications at temperaturesdown to at least -10O0F. Many of these steels contain several alloying elements suchas manganese, nickel,.chromium, molybdenum and vanadium. Several contain small quantities of zirconium or boron, the latter having a potent effect on increasing hardenability. These steels include proprietary grades such as T-1 and N-A-XTRA, amongothers, as well as standard grades such as 'AMs 6434, 4130, 4335, etc.Although the above steels are usable to at least -lOO F, they may, dependingupon steel-making practice, tempering temperature, etc., undergo the tough-to-brittletransition at some temperature between -lOO F and -150'F.For more reliable performance at the lower end of this temperature range, it is necessary to employ somewhatmore highly alloyed quenched and tempered steels such as HY-80 or HY-TUF, both ofwhich are proprietary steels.Low-carbon 3yk nickel steel is widely used in large land-based storage tanks tocontain liquefied gases at temperatures down to -150'F.This steel falls under ASTMA203, Grades D and E, and is subject to impact tests in accordance with.the requirements of A-300.'As shown in Fig: 7, a large number of aluminum, nickel, and titanium-base alloysare suitable for critically stressed applications at temperatures down to -150 F andlower. The high-strength 7079-T6 aluminum alloy may be used down to -200 F, but isnot recommended for lower-temperature applications. In the case of titanium alloys,the 6A1-6V-2Sn-Ti alloy in the heat-treated condition may be used at temperaturesdown to -40 F, and the 16V-2.5Al-Ti alloy may be used down to -lOO F, but neither isrecommended for use at temperatures lower than these.-318-IIIIIIIIIIIIIIIIIII

IIIIIIIIIIIIIIIIIIIIMetals'for use at -320'F.Increasing the nickel content of low-carbon steelprogressively reduces the temperature of transition from duetile to brittle fractureas shown in Fig. 3a. In the normalized and tempered condition, a steel with 9% nickelhas a keyhole-notch Charpy impact energy of 30 ft-lb at -320'F.In the quenched andtempered condition the same steel will show 50 ft-lb impact energy at this temperature.ASTM A353-58 covers the 9% nickel grade and requires the normalized and tempered heattreatmqnt. Revision of current pressure-vessel codes to permit quenched and temperedsteel of this grade for pressure vessels will result in improved reliability of lowtemperature storage tanks.The austenitic stainless steels of the Type 300 series are all suitable for useat -320 F, as is the heat-treatable A-286 stainless steel. Precipitation-hardenablestainless steels of the PI3 series are not recommended for subzero temperature applications since they evidence notch embrittlement at temperatures between OoP and -4OOF.The recently developed maraging steels of the 20% and 25% nickel varieties, withvarious amounts of cobalt, molybdenum, titanium, aluminum and columbium added, exhibitnotch toughness at temperatures down to at least -320 F, and possibly down to liquidhydrogen temperature. The maraging steels are readily formable and weldable, and arehardened by a relatively low-temperature aging at 900 F.A large number of aluminum alloys, including 2024-T6, 7039-T6, 2014-T6, and5456-H343 have excellent resistance to brittle fracture at -320 F, although weld jointsin the 20144'6 alloy tend to exhibit brittle behavior at low temperatures. Other aluminum alloys of the 5000 series aluminum-magnesium type are also tough at -320 F andat.lower temperatures, as are the 6061-T6 and 2219-T87 alloys.Nickel-base alloys are almost all tough at -320 F, as shown in Fig. 7.

LOW TEMPERATURE METALS" A. Hurlich General DynamicslAstronautics San Diego, California Before we consider how low temperatures affect the engineering properties 'of spe- cific metals and alloys, we should review the general effects of low temperature upon the properties of metals. As the tempe